The present invention relates to a linear material used for a stent implanted in the blood vessel of e.g., the coronary artery, and to a blood vessel stent employing this linear material.
If stenosis is formed in the blood vessel of, e.g., the coronary artery, a so-called percutaneous transluminal angioplasty (PTA) is usually applied. The PTA is the operation of inserting a balloon forming portion annexed to the vicinity of the distal end of a catheter in the portion of the blood vessel suffering from stenosis, and of expanding the balloon forming portion to expand the portion of the blood vessel suffering from stenosis to improve the blood flow. Usually, a blood vessel stent is implanted to prevent re-stenosis of the blood vessel in the site of the PTA.
The stent serves to maintain the patency of the blood vessel for the required period, preventing re-stenosis in the site undergoing PTA.
Since it was reported that a metal stent is effective in treating ischemic heart disease, remarkable progress has been achieved in the clinical use of the coronary stent. The coronary stent is highly promising, not only in terms of its prophylatic effect against acute closure, but also in its long-term prevention of restenosis and its therapeutic effect against lesions for which PTCA is considered inadequate. Thus, the coronary stent is widely used in operations such as cardiac intervention.
Comparative clinical trials employing the angioplasty with only a balloon and in employing the angioplasty with both a balloon and a stent have shown that the rates of occurrence of the acute coronary closure and of re-stenosis are both lower when the angioplasty with both a balloon and a stent is employed. Metal stents are reliable in the short and mid-term results as disclosed in many reports. However, the possibility of unexpected events against, for example, the coronary artery has been pointed out in long term tests.
In a metal stent, there is no established standard therapeutic method against re-stenosis in the stent. For example, reapplication of PTCA on occurrence of re-stenosis in the stent (in-stent re-stenosis) may be one method envisaged. However, since the stent is already implanted and remains in the blood vessel, it becomes difficult to expand the balloon, thus obstructing the re-application of PTCA.
Where re-stenosis occurs over a long area or in multiple branches, a frequently-used method is that of implanting a number of metal stents side-by-side. By surgical thoracotomy, a by-pass operation for preserving blood flow by connecting a by-pass to the blood vessel so as to avoid the site of stenosis is an effective technique against in-stent re-stenosis. However, some metal stents do not show up easily in angiography and, if a metal stent remains implanted in the target portion where the bypass is to be connected, the bypass operation itself has to be abandoned. This is a significant burden for the patient.
Currently, investigations are underway as to the compatibility of blood flow and metal. Since metal is hydrophilic and tends to form the thrombus, the thrombus-forming properties of the metal stent pose a serious problem. Thus, for the purpose of prevention of thrombotic closure in the stent-implanted portion, a concentrated anti-thrombotic therapeutic operation is indispensable. However, there is always the associated risk of further hemorrhagic complications.
For these reasons, implanting a metal stent which will remain permanently in the body poses a problem.
The main objective of implanting a stent is to avoid acute coronary closure and to reduce the frequency of the occurrence of re-stenosis. It is reported that, since acute coronary closure and re-stenosis are phenomena that relate each other for a certain period, they require only temporary treatment. It is, therefore, desirable that the function of the stent may be maintained only for a required period such that the stent will not remain in the living body as foreign substance once its role has been fulfilled. In particular, since the increase in the rate of re-stenosis wanes after about six months, the function of the stent needs to be maintained during this six-month time period.
In view of the physical functions, the following properties are required of the stent:
(a) mechanical properties: such that, as the structural element, the stent needs to possess sufficient mechanical properties to keep the blood vessel open for a specified period of time; and
(b) expansion/contraction properties: such that the stent diameter needs to be decreased during delivering to the target legion and to be increased up to a desired diameter at the target site of lesion; and
(c) delivering capability: such that, the stent needs to be correctly implanted in the target legion of the coronary artery without migration, kinking, distortion or breakage.